Cycloaddition reactions of cyclic ketene-N,S-acetals with 1,2,4,5-tetrazines

Reaction of 3,6-diphenyl-, 3,6-bis(2-pyridyl)- and the unsubstituted 1,2,4,5-tetrazine with 4,5-dihydro-1-methyl-2-(methylthio)pyrrole ( 2 ) and 1-raethyl-2-(methylthio)-4.5,6,7-tetrahydroazepine ( 3 ) gives 4,7-di-R-2,3-dihydro-1-methylpyrrolo[2,3-d]pyridazine ( 4 , R = phenyl, 2-pyridyl, hydrogen) and 6,9-di-R-1-methyl-2,3,4,5-tetrahydropyridazino[4,5-6]azepine ( 5 ), R = phenyl, 2-pyridyl, hydrogen), respectively, in reasonable to good yields. The compounds 4 (R = phenyl, hydrogen) are converted into their corresponding 1-methylpyrrolo-[2,3-d]pyridazines 6 by reaction with potassium permanganate in butanone. Reaction of 3-phenyl-1,2,4,5-te-trazine with 2 and 3 leads to the exclusive formation of the 7-phenyl isomer 4d and 9-phenyl isomer 5d , respectively, indicating that the cycloaddition is regiospecific. The mechanism is discussed.